Orthogonal frequency division multiplexing (OFDM) is an effective multicarrier modulation technique for mitigating the effect of inter-symbol interference (ISI) in frequency-selective wireless channels. In wireless OFDM systems, the use of differential phase-shift keying (DPSK) eliminates the need for channel estimation at the receiver. However, DPSK limits the number of bits per symbol and results in a 3 dB loss in signal-to-noise ratio (SNR).
If the channel is estimated at the receiver, coherent detection may be used. This permits the use of more efficient multi-amplitude signaling schemes. If the channel state changes slowly, reference pilot symbols or decision-directed channel tracking techniques may be used. On the other hand, if the channel state can change significantly from one symbol to the next due to high Doppler frequency, channel estimation within a single OFDM symbol may be required. This can be achieved using L pilot tones equally spaced across the N sub-channels in the frequency domain.
OFDM systems usually use coding and interleaving across sub-channels to exploit frequency diversity in frequency selective channels. Blind channel estimation techniques allow higher data rates because there is no overhead caused by training symbols. Most of the traditional blind channel estimation techniques, however, ignore the coding information and thus require a large number of OFDM symbols to achieve a sufficiently accurate estimate of the channel. This requirement not only introduces a significant latency in the system, but also limits these techniques to slowly varying channels.
Therefore, there is a need in the art for improved coded orthogonal frequency division multiplexing (OFDM) receivers. In particular, there is a need for improved OFDM receivers that can perform channel estimation within as little as a single symbol and using the minimum possible number of pilots, if any.